1. Field of the Invention
The present invention relates to an apparatus for reducing particulate matter. More particularly, the present invention relates to a particulate matter reducing apparatus which can collect, burn and reduce the particulate matter contained in an exhaust gas of a diesel engine. For example, the present invention relates to a reducing apparatus which can be additionally mounted later on an existing car which is being used.
2. Description of the Art
An exhaust gas of a diesel engine contains CO, HC, NOx, Particulate matter and the like. If directly discharged to the open air, these are harmful to the human body and the environment. It is therefore an important theme to reduce these harmful substances.
The present invention therefore relates to a particulate matter reducing apparatus for reducing the particulate matter among these harmful substances.
FIG. 5 is an explanatory cross-sectional view of a conventional particulate matter reducing apparatus and the like of this kind. An exhaust gas purifier 3 is connected to an exhaust pipe 2 of exhaust gas 1 discharged from a diesel engine. This exhaust gas purifier 3 is provided with a purifier 5 and a particulate matter reducing apparatus 6 within an outer cylindrical casing 4 in that order.
The purifier 5 on the upstream side is composed of a honeycomb core of which the wall of each cell is coated with an oxidation catalyst 7. The purifier 5 burns and reduces CO and HC in the exhaust gas 1 by oxidation and oxidizes NO to NO2. The particulate matter reducing apparatus 6 on the downstream side burns and reduces the particulate matter in the exhaust gas 1 by oxidation.
As is well known, the diesel engine is in a lean and excess air condition, i.e., in an excess oxygen condition in an air fuel ratio compared with a gasoline engine, wherein NO in the exhaust gas 1 can hardly be deoxidized. In addition, NO and the particulate matter in the exhaust gas 1 are in a trade-off relation, wherein the higher the burning temperature of the diesel engine, the higher the former and the lower the latter.
As the particulate matter reducing apparatus 6 for burning and reducing the particulate matter in the exhaust gas 1 of the diesel engine, there is an apparatus of a type in which an oxidation catalyst us used. However, a high-performance type using a filter 8 as shown in FIG. 5 is widely used.
The particulate matter reducing apparatus 6 using this filter 8 is also referred to as a diesel particulate filter (hereinafter referred to as “DPF”). In the particulate matter reducing apparatus 6, the particulate matter is first collected on a filter 8, then burned and reduced. By burning and eliminating the particulate matter, the filter 8 can be regenerated.
As the particulate matter reducing apparatus 6 of such a DPF type, various kinds of high-density porous filters 8 have been developed and used.
It is structurally typical to coaxially house a filter 8 of a wall-flow type in an outer cylindrical casing 4 in the same diameter and cross-sectional area as the outer casing 4. This filter 8 is provided with many air holes divided by many thin walls in the flow direction and the inlet and outlet of each air hole is alternately sealed. When the exhaust gas 1 introduced into each air hole passes through the numerous pores of the thin wall, the particulate matter contained in the exhaust gas 1 is first collected on the pores of the thin wall, then, reduced by burning.
The particulate matter consist of about the size of several μm in a primary particle condition, but these are usually interconnected in a secondary particle condition of about the size of several hundred μm. Since the diameter of numerous pores of the thin wall of the filter 8 is small with about the size of 10 μm˜100 μm, almost all the particulate matter has been collected.
As the material of such a thin wall of a wall-flow typed filter 8, cordierite (which is made by hardening Al2O3 and Si2O3 by a binder), SiC, or various other ceramics are typically used.
It is to be noted that the particulate matter reducing apparatuses 6 of various other DPF types have also been developed or used. For example, a particulate matter reducing apparatus 6 has been developed or used, in which NO in the exhaust gas 1 is oxidized to NO2 by an oxidation catalyst 7 and the obtained NO2 promotes the burning of the particulate matter collected on the filter 8.
A filter of a wall-flow type made of ceramics, a filter made of foamed ceramics, a filter made of ceramic fiber, a filter of wire-mesh structure or the like is available as the filter 8. Such a filter has been coaxially housed in an outer cylindrical casing 4 of the same diameter and cross-sectional area as the outer casing 4.
A conventional wall-flow typed particulate matter reducing apparatus 6 is disclosed in the following patent documents 1 and 2:
[Patent Document 1] Specification of U.S. Pat. No. 4,329,162
[Patent Document 2] Specification of European Patent No. 31348
However, the following problems are pointed out concerning such a conventional particulate matter reducing apparatus 6.
<First Problem>
First, conventional particulate matter reducing apparatuses 6 of a DPF type have adopted different methods for collecting a large amount of particulate matter on the filter 8, respectively.
As described above, the typically used wall flow type filter 8 consists of a method whereby almost all the particulate matter has been collected, wherein the collection rate was 90% or more. Even the other filters 8 have adopted methods of a high collection rate in which the collection rate is 50% or more.
In use of such a filter 8, the following problems have been pointed out. Since particulate matters of more than the allowable amount or collection limit have been collected on the filter 8, the filter 8 is readily clogged with the collected particulate matter. For regeneration, the filter 8 must be cleaned up with extremely high frequency. In particular, clogging has intensively and biasedly occurred in the vicinity of an intake surface section on the upstream side of the filter 8.
Further, problems have been pointed out in that the filter 8 must be cleaned up, for example, once a day, the cleaning takes a lot of time and it is troublesome, and regeneration of the filter 8 is not easy.
For regeneration of such a filter 8, a method for incorporating an electric heater into the filter 8, a method for taking the filter 8 out and bringing it into a heating furnace, a method for alternately regenerating the heater 8 or the like has been developed and used to burn and eliminate the particulate matter collected in large quantity. However, there was a drawback in that these methods cost a great amount for the equipment and the running cost becomes high.
<Second Problem>
Second, in the conventional particulate matter reducing apparatus of a DPF type, a high-density porous filter 8 is coaxially housed in the outer cylindrical casing of the same diameter and cross-sectional area as the outer casing. In this case, the area of the intake and exhaust surface of the filter 8 is small. For example, the suction and exhaust surface have the same area as the cross-section of the outer cylindrical casing 4. In use, the conventional filter 8, structurally, has a large resistance to the flow of the exhaust gas 1. Thus, a larger resistance is generated by friction and the like and there is a large pressure loss in the exhaust gas 1.
In addition, as described above, the conventional filter 8 has shown a high collection rate of, for example, 90% or more and at least, 50% ore more. Thus, the resistance to the flow of exhaust gas 1 becomes even larger because of the collected particulate matter. In particular, the resistance and the pressure loss have become larger as the collection advances to come close to the clogging conditions. This becomes intensively and biasedly marked in the vicinity of the intake surface section on the upstream side of the filter 8.
It has been pointed out that the back pressure of the exhaust gas 1 within an exhaust pipe 2 on the upstream side of the filter 8 rises according to generation of the resistance and the pressure loss in such a filter 8 and the increase of back pressure has a bad influence on an engine disposed further upstream. Namely, it has been pointed out that the back pressure increase puts an excessive load on the engine to excessively increase the driving torque, thereby worsening the fuel consumption and increasing the incidence rate and the content by percentage of the particulate matter in the exhaust gas 1.
<Third Problem>
Third, the filter 8 of the conventional particulate matter reducing apparatus 6 of a DPF type has shown a high collection rate as described above.
It has been pointed out that the temperature of the filter 8 rises suddenly because the particulate matter collected and retained on the filter 8 in large quantities catches fire and burns at one time, and the filter 8 is in danger of melting by the high temperature or of being damaged by heat. In the case of the filter 8 made of the cordierite described above, the binder easily melts away. Further, such a problem intensively and biasedly occurs in the vicinity of the suction surface section on the upstream side of the filter 8.
In this manner, in use, the filter 8 easily melts or is damaged by heat. Sometimes, the filter 8 becomes unusable, for example, in about a week. Thus, it has been pointed out that it is difficult to regeneratively use the filter 8, the filter 8 has a problem in durability, its life is short, and it is difficult to bear the cost.
A particulate matter reducing apparatus of the present invention was developed to solve the foregoing problems of the conventional examples in view of such actual conditions and is characterized in that one or more filters have been adopted in combination with each of the following:
That is, a wire mesh structure; a short column shape with one central through hole; a coaxial arrangement within an outer cylindrical casing; a pair of shielding plates with one or more air holes; the diameter and filling density of a wire; a low collection rate; a wire of which the main component is Fe; an oxidation catalyst; a fuel borne catalyst, etc.
It is therefore an object of the present invention to provide an improved particulate matter reducing apparatus in which first, a filter is barely clogged and the trouble of cleaning can be saved; second, increase of the back pressure can be controlled to avoid a bad influence on a diesel engine; third, the filter is in no danger of melting or being damaged by heat; fourth, these conditions can be readily realized; and fifth, a high reduction and purification rate can be attained.